The concept of transmitting several television channels through optical fiber using analog intensity modulation of the output of a semiconductor laser diode has recently been receiving considerable attention. As proposed in the prior art, this would involve transmission of multi-channel amplitude modulated-vestigial side band (AM-VSB) signals, as used in present day cable television (CATV) systems, in an optical fiber transmission medium. Such an arrangement would be useful in a CATV trunk system or in a fiber-to-the-home network. Optical fiber transmission systems that use frequency division multiplexing overcome compatibility problems and have advantages such as simplicity of design, reduced bandwidth requirements for lightwave components, and much lower costs, as compared with optical time division multiplex (TDM) systems.
The wide bandwidths of semiconductor laser diodes and optical fibers make analog sub-carrier modulation an attractive technology. Several signals at different sub-carrier frequencies, each signal representing one of the television channels to be multiplexed, are summed and applied concurrently to the input of the laser device. The input information signal is a set of frequency-modulated sub-carriers at different frequencies, e.g., frequencies .omega..sub.0, .omega..sub.1, .omega..sub.2, . . . . The resulting laser injection current comprises a dc bias level plus the set of frequency-modulated sub-carrier signals. The magnitude of the optical output power from the laser fluctuates with the magnitude of the laser injection current. The resulting sub-carrier frequency division multiplexed (FDM) optical output signal is applied to an optical fiber for transmission over an extended distance. After transmission through the fiber the optical signal is detected by appropriate means, e.g., a PIN diode, and the resulting electrical signal is processed by conventional means to recover the individual signals. See, for instance, R. Olshansky et al., Electronics Letters, Vol. 23(22), pp. 1196-1197 (1987).
Multi-channel amplitude modulated signal transmission requires special limitations on the power, the non-linearity, and the intensity noise of the transmitting laser diode. For adequate system performance, laser output light intensity must be a very nearly linear function of the laser drive current under large-signal modulation. Strict limitations on laser nonlinearity are required because of the wide dynamic range of the National Television Systems Committee (NTSC) standard video format. Exemplarily, in the NTSC standard video format, the ratio of the magnitude of the carrier to the magnitude of the total third order intermodulation distortion products at the carrier frequency must be less than -60 dBc. Similarly, the peak second-order distortion, i.e., the sum of several tens of two-tone products (or the ratio of the carrier to the largest composite second-order peak), must be less than -60 dBc. To obtain such high signal quality in view of the large number of distortion products, the transmitting laser light-versus-current characteristic must be extremely linear.
In a system that uses frequency division multiplexing any nonlinearity in the laser diode characteristic will result in intermodulation noise. Laser nonlinearities create energy transfers from the applied sub-carrier frequencies to, among others, those frequencies that are the sum and difference frequencies of all of the pairs of applied signal frequencies. Such energy transfers cause undesirable intermodulation distortion and interference, both of which can limit the performance of the transmission system.
There are several known causes of nonlinearity in semiconductor laser diodes. Some of the causes of nonlinearity are high frequency relaxation oscillations, low frequency heating effects, damping mechanisms, optical modulation depth, leakage current, gain compression, and nonlinear absorption. The resulting effect of the distortion and interference is a degradation in the signal-to-noise ratio for the signal, as received further along in the system.
An experimental sub-carrier frequency division multiplexed, optical communication system having sixty frequency-modulated channels in the 2 GHz to 8 GHz band has been operated with a 56 dB weighted signal-to-noise ratio. Other arrangements using microwave carriers for subscriber loop transmission have put (1) five frequency-modulated video channels in the 150 MHz to 300 MHz band and (2) ten frequency-modulated video channels in a C-band satellite signal in the 4.9 GHz to 5.2 GHz band.
The currently most attractive scheme for multiplexing multiple video channels onto a continuous-wave laser output involves amplitude modulated-vestigal sideband signal multiplexing. Some previously available semiconductor lasers can exhibit distortions approaching the required low levels. However, typically only a small fraction of a given batch of otherwise suitable lasers meet the distortion requirements, requiring extensive noise measurements to identify those lasers that have sufficiently low distortion. U.S. patent application Ser. No. 420,867 U.S. Pat. No. 5,034,334, incorporated herein by reference, discloses a method of producing lasers that includes a simple technique for identifying lasers that will have low distortion and thus may be suitable for use in a multichannel analog fiber communication system. Such a system is disclosed in U.S. patent application Ser. No. 420,849, U.S. Pat. No. 5,020,049 also incorporated herein by reference.
However, even though there now exists a method that permits easy identification of lasers that have low distortion, the number of lasers on a given wafer that have requisite low distortion typically is quite low. Such low yield of course is highly undesirable since it results in relatively high unit cost of acceptable lasers. A laser design that can result in increased yield of low distortion lasers thus would have substantial economic significance. This application discloses a distributed feedback (DFB) laser having novel design features that can result in increased yield of lasers acceptable for use in multichannel analog communication systems.
Various aspects of DFB laser design have previously been considered with a view towards optimizing performance of such lasers in digital applications, including coherent optical fiber transmission systems. This work generally aimed at, inter alia, relatively high slope efficiency ("slope efficiency" is defined herein as the maximum value of dL/dI of the laser, where L is the radiation output power at the front facet of the laser, and I is the laser drive current) and, consequently, relatively high output power asymmetry between the front and back facets of the lasers. Other important design criteria for digital laser applications include spectral purity (or "side mode suppression ratio"), chirp, and linewidth. To optimize the performance of devices in these respects the use of various facet coatings and phase shifts have been considered.
For instance, N. Eda et al., J. of Lightwave Technology, Vol. LT-3(2), pp. 400-407 (1985) show that the optimal value of the coupling parameter KL for a AR/AR phase shifted DFB laser is about 2, and suggest use of a lower value of K in the front of the laser to improve the front facet output efficiency (K is the grating coupling constant and L is the length of the laser "cavity", i.e., the distance between the front and back facets. By "AR/AR" is meant that the laser has both facets anti-reflection coated).
H. Wu et al., Applied Physics Letters, Vol. 52(14), pp. 1119-1121 (1988), discloses that in AR/AR DFB lasers relatively large values of KL lead to line broadening, and also discloses that a HR/AR laser has a more uniform field distribution than a AR/AR laser, resulting in reduced longitudinal spatial hole burning. ("HR" designates a facet with high reflection coating). On the other hand, L. D. Westbrook et al., IEEE J. of Quantum Electronics, Vol. QE-21(6), pp. 512-518 (1985), discuss the experimental determination of KL and report observation of narrow linewidths even for relatively large (e.g., 2, 4 and 4.8) values of KL.
C. H. Henry, IEEE J. of Quantum Electronics, Vol. QE-21(12), pp. 1913-1918 (1985) shows that in a HR/AR DFB laser KL of about 1 is optimal, and KL.ltorsim.2 is needed in order to attain high mode selectivity and quantum efficiency, and good insensitivity to reflections.
H. Soda et al., IEEE J. of Quantum Electronics, Vol. QE-23(6), pp. 804-814 (1987) give experimental results for AR/AR phase-shifted DFB lasers, and disclose that moderate coupling (KL.about.1.25) is optimum to maintain high mode selectivity above threshold. The phase-shift was symmetrically placed. Best yield of acceptable lasers was also obtained for KL of about 1.25.
K. Utaka et al., IEEE J. of Quantum Electronics, Vol. QE-22(17), pp. 1042-1051 (1986), teaches that in AR/AR phase shifted DFB lasers placement of the phase-shift toward the front facet of the cavity improves efficiency of power extraction, and the optimum structure in terms of wavelength selectivity is one having the phase-shift at the center of the cavity, with both end reflectivities being zero.
Y. Kotai et al., Electronics Letters, Vol. 22, pp. 462-463 (1986) teach that the output power asymmetry can be enhanced by placing the phase-shift near the front of the laser. See also F. Favre, Electronics Letters, Vol. 22(21), pp. 1113-1114 (1986), and S. McCall et al., IEEE J. of Quantum Electronics, Vol. QE-21(12), pp. 1899-1904 (1985).